Also added the allocate method to DevicePointer and UniquePointer for
consistency. Added missing free() method to UniquePointer.

The original 'modified' flag was not shared between instances of the
smart pointer, so theoretically the same data object might have been
synchronized more than once or even not at all.

Another problem with the detection was that e.g. access through the
modifyData method does not imply modification of the object's class
data. For example if the object is TNL::Vector, we need to go through
modifyData to be able to modify the vector data, but synchronization is
not needed since we go through another pointer. Another example is
repeated binding of the same object, e.g. bindDofs in the Problem
classes.

This is necessary if the pointer type is const-qualified.